The present invention generally relates to a developing apparatus for use in an image forming arrangement based on electrophotography and more particularly, to a developing apparatus of the above type including a plurality of developing units.
Commonly, in an image forming arrangement based on the electrophotographic process, it is so arranged that toner is supplied from a developing apparatus onto a surface of a photosensitive member on which an electrostatic latent image is formed so as to develop the latent image into a visible image. In the image forming apparatus of the above described type, in the case where a color image is to be formed, images for respective colors for three primaries in the subtractive mixture are formed on the surface of the photosensitive member so as to be composed or combined. For correctly effecting such color composition, it is necessary to avoid mixture of respective colors in the developing process for each color, and in the conventional color image forming arrangement, developing apparatuses are formed into units for the respective colors, with each of the developing units being provided with a separate magnet roller for supplying toner onto the surface of the photosensitive member.
In the developing apparatus having a plurality of developing units as described above, it has been a practice to independently detect the amount of toner accommodated in each developing unit, thereby to effect a display to urge toner replenishment with respect to the developing unit showing the accommodated toner amount below a predetermined value, so that an operator recognizes said display and replenishes the toner in the color accommodated in the developing unit with respect to said developing unit which has decreased in the accommodated toner amount.
However, since the toner consuming amounts in the respective developing units are not generally uniform, it is seldom that the accommodated toner amounts simultaneously fall below the predetermined value in the plurality of developing units, and thus, it is required to independently replenish toner with respect to each of the developing units, resulting in such an inconvenience that frequency for the toner replenishments is undesirably increased and more complicated, with a consequent reduction in the working efficiency.